Known examples of semiconductor diodes for dynamic single-mode oscillation are distributed feedback (DFB) semiconductor diodes and distributed Bragg reflector (DBR) semiconductor diodes. These semiconductor diodes each include a waveguide structure containing a grating with a wavelength-selecting function. A refractive index step waveguide structure usually includes high-refractive index cladding layers and a low-refractive index waveguide layer sandwiched therebetween.
These semiconductor diodes include first cladding layers having a first conductivity type, active layers, grating layers, second cladding layers having a second conductivity type, and contact layers arranged in that order as discussed in, for example, Japanese Laid-Open Patent Publication No. 2000-357841. The grating layers each include a GaAs layer and AlGaAs layers disposed therein.
A procedure for forming a grating layer included in such a semiconductor diode is as described below. A first GaAs layer, an AlGaAs layer, and a second GaAs layer are deposited on an active layer in that order, the first GaAs layer and the AlGaAs layer being included in a lower portion of the grating layer, the second GaAs layer being included in an upper portion of the grating layer. A dielectric mask is provided on the second GaAs layer. The second GaAs layer and the AlGaAs layer are etched through the dielectric mask, whereby an irregular structure is formed. The irregular structure has recessed portions arranged in the first GaAs layer and protruding portions including portions of the AlGaAs layer and portions of the second GaAs layer. GaAs is deposited on the irregular structure such that the recessed portions of the AlGaAs layer are filled with GaAs, whereby the grating layer is formed. The upper surface of the second GaAs layer needs to be flat. This is because a flat second cladding layer is formed later.
In order to planarize the irregular structure during the formation of the grating layer, the second GaAs layer needs to be grown so as to have a large thickness. When the recessed portions are filled with GaAs, which is an As-containing material, crystals of GaAs are grown on the protruding portions; hence, the shape of the irregular structure is maintained. In order to planarize the upper surface of the grating layer, GaAs needs to be further deposited on the irregular structure after the filling of the recessed portions with GaAs until irregularities due to the irregular structure disappear. Therefore, the second GaAs layer has an increased thickness.
When the second GaAs layer has an increased thickness, the field of light guided in the active layer is attracted toward the second GaAs layer because the grating layer overlies the active layer and the second GaAs layer has a large refractive index. This reduces the effect of confining light in the active layer. A reduction in the effect of confining light in the active layer causes a reduction in effective gain and an increase in oscillation threshold current. This may deteriorate the oscillation properties.
In order to prevent the reduction of the effect of confining light in the active layer, the thickness of a re-grown layer made of GaAs, that is, it may be desirable to prevent the thickness of the second GaAs layer from increasing and it may be desirable to flatten the upper surface of the grating layer.